This invention is directed to cathodes useful in the electrolysis of water containing an alkali metal hydroxide electrolyte or the electrolysis of aqueous solutions of alkali metal halide salts. More particularly it is directed to cathodes having a coating of foraminous nickel or cobalt formed by melt spraying and leaching that exhibits in those electrolytic processes reduced hydrogen overvoltage and good durability and life span.
In the electrolysis of water or aqueous alkali metal halide solutions in electrolytic cells having a diaphragm or membrane separator, the working voltage required comprises, in the main, the decomposition voltage of the compound being electrolyzed, the voltages required to overcome the ohmic resistances of the electrolyte and the cell electrical connections, and the potentials, known as "overvoltages," required to overcome the passage of current at the surfaces of the cathode and anode. Such overvoltage is related to factors as the nature of the ions being charged or discharged, the current per unit area of electrode surface (current density), the material of which the electrode is made, the state of the electrode surface (e.g. whether smooth or rough), temperature, and the presence of impurities in either the electrode or electrolyte. While various theories have been advanced to explain overvoltage, at the present time knowledge of the phenomenon is almost wholly empirical: it being observed that a characteristic overvoltage exists for every particular combination of discharging (or charging) ion, electrode, electolyte, current density, and so forth.
Because of the multi-million-ton quantity of chloro-alkalies and water electrolyzed each year, even a reduction of as little as 0.05 volts in working voltage translates to meaningful economic savings especially with today's constantly increasing power costs. Consequently, the electrochemical industry has sought means to reduce the voltage requirements for such electrolytic processes. One means that has received attention is the provision of cathodes that have reduced hydrogen overvoltage: as, for example, cathodes made of or coated with sintered nickel or stell powder, or cathodes having particular metal- or metal alloy-coated surfaces. See, for example, U.S. Pat. Nos. 3,282,808, 3,291,714 and 3,340,294. However, such cathodes have not been adopted, it seems, to any significant degree, and steel cathodes still predominate. While the reasons for such nonuse are not clear, it may be that the costs of some, i.e., cost of producing and life span, versus realizable power savings, are unattractive. Another reason may be the inability of others to be readily fabricated. For example, sintered metal coatings are difficult to apply uniformly, especially to cathode substrates having irregular surfaces such as expanded or woven steel mesh.